Canadian Rockhound: Sheldrickite

Sheldrickite
Abstracted by Malcolm Back

Sheldrickite, ideally Na Ca₃(CO₃)₂F₃^.H₂O, is a newly described mineral species from Mont Saint-Hilaire, Quebec, and was originally given the designation UK90. The cotype specimens were collected in 1991 by staff from the CMN and by Gilles Haineault. At present it is considered to be very rare; only approximately 60 mg. of material are known to exist.

The specimens were collected in the Poudrette quarry from a marble xenolith in nepheline syenite, close to a large unit of hornfels. Sheldrickite is intimately associated with shortite, the crystals being found in a cavity between shortite crystals and as flakes in thin seams between crystals of shortite. Other associated species include pectolite, microcline, polylithionite, arfvedsonite, aegirine, calcite, fluorite, and minor molybdenite, leucosphenite, thenardite, thermonatrite, sphalerite, galena, schairerite and kogarkoite. The mineral assemblage is thought to be a late-stage hydrothermal infilling.

Sheldrickite has two distinct habits: (1) it occurs as a 1 x 1 x 2 mm aggregate of blocky twinned crystals with individuals up to 0.1 mm, and (2) more commonly as radiating thin white silky flakes to fibrous masses up to 2 mm wide. The mineral is colorless to white with a vitreous lustre. It is brittle with an uneven fracture, and has a good {001} parting. Sheldrickite is relatively soft (Mohs hardness 3) and does not fluoresce. It is uniaxial positive, _ = 1.583(2) and e = 1.563(4)(for l = 589 nm) and nonpleochroic. Sheldrickite is trigonal, space group P3₂ , a 6.718(3)Å, c 15.050(4) Å, V 588.3(3) Å³, and Z = 3.

Source:

Grice, J. D., Gault, R. A., and Van Velthuizen, J. 1997. Sheldrickite, A New Sodium-Calcium-Fluorocarbonate Mineral Species from Mont Saint-Hilaire, Québec. Can. Min. 35:181-187.

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